Conventionally, there has been known a battery state detecting device in which a battery state detecting unit disposed for each of individual batteries constituting an assembled battery detects a battery state of each of the individual batteries, and the detected battery state is transmitted from the battery state detecting unit to an ECU for management (see, PTL 1). In the battery state detecting device, the battery state detecting unit is started in response to a battery state detection request received from the ECU for management and detects the battery state of the corresponding individual battery. After the detected battery state is transmitted to the ECU for management, the battery state detecting unit is held in a start standby state (sleep state).